1. Field of the Invention
The invention relates to method for manufacturing ceramic materials. In particular, this invention relates to a method for microwave heating and drying ceramics, and more particularly to a method for variably and efficiently controlling the power and the heating/drying rate.
2. Discussion of the Related Art
Conventional heating or drying typically comprising convectional or a combination of convectional and radiative gas or electric resistance heating, is commonly used in the manufacturing of ceramic materials. However, the slow heating rate and poor temperature control associated with these conventional heating methods results in a high energy consumption and inconsistent product quality. Furthermore, utilization of these two modes of heating typically result in thermal differences within the ceramic body, due to the fact that these two heating modes are applied only to the surface and rely on thermal conductivity of the ceramic body to effect the temperature beneath the surface to the center of the piece.
Industrial heating by microwave radiation has been successfully used to accelerate the drying of traditional ceramics. In comparison with convectional heating, microwave heating provides a higher heating rate, where there is sufficient absorbtion, with better temperature control, and thus results in lower energy consumption and potentially better quality products. Furthermore, the utilization of microwave energy delivers a uniform application of the energy to the ceramic article, rather than to the article surface, as is the case for the aforementioned convectional and radiative modes of heating. Lastly, microwave heating is much faster than conventional drying because the ceramic body is heated directly through the interaction of the microwave energy with the ceramic body.
Although microwave heating is faster and more efficient than conventional modes such as convectional and radiative heating, standard microwave heating typically involved controlling the amount of microwave energy utilizing a constant power setpoint to determine the amount of microwave to which the ceramic body is subject to. Typically, this power output is set at some value such that ensures that the reflected power never exceeds the manufacturers specification; i.e., a power output assuming a constant load and material dielectric characteristics. The unfortunate result of utilizing this conventional method of controlling microwave heating is that any variation in the amount of mass of material in the microwave (loading) or variations in the dielectric characteristics of the load or variations in geometries and densities (i.e., varying heights or lengths), are not compensated for. As a result, the microwave heating is inefficient because the power input at various times during heating is far below that which the ceramic load is capable of handling. This inefficiency forces current drying processes to be tailored to the capabilities of the drying equipment; i.e., increasing residence/drying time of the article to be dried, thereby limiting the throughput of the material to be dried and, in many cases producing defects in the material being processed. In sum, conventional control of microwave drying forces the equipment to be utilized at power levels that are either inefficient or result in shortened magnetron tube life due to the operation in high ranges of reflected power.
PCT Application WO 93/12629 discloses a method of preventing thermal runaway and increasing microwave efficiency in the heating of ceramic materials. Provided therein is an apparatus that includes a microwave resonant cavity, a magnetron for generating microwaves and a means for continuously controlling the power of the magnetron, with the means comprising a temperature control system, specifically an optical fibre pyrometry system, and a solid state control circuit. This "means for controlling the microwave power" controls the power of the magnetron in response to any difference between a set value of the temperature of the ceramic items and a measured value of the of ceramic items temperature. Although this apparatus is capable of providing a variable power source of microwave energy in response to a temperature parameter and thus control the temperature of the ceramic to prevent thermal runaway, it is still deficient. Specifically, the use of a temperature control system utilizing optical fibre pyrometry measures only the external surface temperature of the ceramic and is not capable of measuring the temperature of the ceramic core. As such, high delta temperatures develop between the skin and core of the piece, ultimately producing stresses in the product which result in uneven drying and potential defects such as cracking, blistering and/or fissuring